On the emergent spatial structure of size-structured populations: When does self-thinning lead to a reduction in clustering?

Abstract

It is widely held that if competition is important, adult or mature plants should be less-clustered than those in smaller size-classes. Self-thinning, the competition for limited resources, should remove individuals that are nearby neighbours, restricting recruitment of adults around other adult trees and resulting in a decrease in aggregation with size-class. 2. There is good empirical evidence for a reduction in clustering with increase in size-class, and the pattern is strongest for richly abundant species. However, few dynamical models have been developed to explore the relationship between the emergent spatial pattern of adults and juvenile plants and the underlying processes of neighbourhood competition and dispersal. In particular is it correct to assume that increased clustering with size-classes is inconsistent with self-thinning? 3. The hypothesis that self-thinning leads to less-clustered adult trees is assessed by analysing a deterministic approximation to a spatially explicit individual-based model that incorporates two size-classes, local dispersal, and neighbourhood competition that increases death rates and retards the growth of the smaller trees. 4. Analyses show that if recruitment of adults is limited by slow growth rates, low fecundity, or high juvenile-juvenile competition, then adults may well be more clustered than juveniles. This means an increase in aggregation with size-class is consistent with self-thinning processes. 5. The models support previous empirical results in that the numerically dominant species are more likely to show a reduction in clustering with size-class than species with a lower abundance. The models suggest rare species should show more adult clustering and less of a reduction in aggregation with size-class due to a numerical decrease in conspecific interactions. 6. Synthesis. The results show clearly that the Janzen-Connell hypothesis and self-thinning in general are consistent with adults being more clustered than juveniles, even when competitive interactions are quite strong. The pattern of relative clustering in juveniles and adults may therefore be indicative of adult recruitment rates, with weak decreases in aggregation with size-class caused by low fecundity, poor dispersal or slow growth, and strong decreases generated by high fecundity, good dispersal and/or fast growth rates. © 2009 British Ecological Society.